Ribonucleotide reductase from Lactobacillus leichamannii partially purified by published procedures is brought to homogeneity by affinity chromatography and is being studied as an example of a regulatable monomeric enzyme with an allosteric and a catalytic site on a single polypeptide chain. The conformational transitions induced when modifiers bind to the allosteric site will be studied by high precision sedimentation, CD, ORD, fluorimetry, difference spectroscopy and NMR. The amino acid residues at the two sites will be investigated by chemical modification and the location of these residues in the primary sequence will be determined. Conditions for crystallizing the enzyme will be investigated and suitable crystals will be examined by X-ray diffraction with a view to determining the three dimensional structure of the native enzyme and of the modifier-enzyme complex. The effect of modifier-induced conformational transitions will be investigated by substrate binding studies and by measurement of rates of partial reactions. Various other monomeric enzymes that aggregate as a result of modifier-induced conformational transitions will also be examined to determine their suitability for a detailed study of the same kind which would, in addition, examine how the transition causes aggregation.